1. Field of the Invention
The present invention concerns a method of determining the pretilt angle of the liquid crystal molecules of a liquid crystal display (LCD) provided with top and bottom cover plates.
2. Description of the Prior Art
Liquid crystal displays are being used on an increasing scale in many fields of technology. They are provided with top and bottom cover plates, the liquid crystal being contained between these plates. Each cover plate consists of several different layers, for example, a polarization filter layer, a glass plate, an electrode layer, and an orientation layer. The latter layer is subjected to a special treatment during manufacture in order to enable it to impress a particular alignment or orientation upon the liquid crystal molecules. The orientation layer consists preferably of organic polymers (polyimides, polyphenyls) and, during the manufacturing process, it is "rubbed", especially with a roller, in order to confer upon it the property of orienting the liquid crystal molecules. According to the particular rubbing direction, it is possible to produce liquid crystal molecule patterns with different helix structures. The term "helix structure" is to be understood as referring to the twist of the liquid crystal molecules that constitute a row between the cover plates of the liquid crystal cell. The liquid crystal molecules are essentially rod-shaped and their longitudinal alignment is more or less parallel to the planes of the cover plates. Inasmuch as these molecules have an angle of incidence (also known as angle of attack or pretilt angle) with respect to the cover plate plane, they do however deviate from this basic parallelity. The said pretilt angle stabilizes the helix and is therefore a desirable feature. In a socalled TN cell the twist angle amounts to 90.degree.. However, such a cell has the disadvantage that the display can be clearly recognized only within a relatively narrow viewing angle Liquid crystal displays with twist angles of 220.degree. are described as "supertwisted nematic" (STN) and can be read over a considerably larger viewing angle. Socalled EVA (extended viewing angle) displays are even further improved as far as the viewing angle is concerned. They have a twist angle of 270.degree..
The above remarks make it clear that either during the manufacture or the use of a liquid crystal display (LCD) it may become necessary to determine the angle of attack (pretilt angle) of the liquid crystal molecules. Various methods to this end are known in the state of the art. In one known laser method the pretilt angle is determined by means of an interference measurement. A method based on the Frederickz effect is also known and determines the angle of incidence with the help of a magnetic field. Both known methods have the drawback that very complicated equipment is needed to carry them out, so that they both involve considerable cost.